A membrane is a layer of material which serves as a selective barrier between two phases and remains at least partially impermeable to certain particles, molecules, or substances when exposed to the action of a driving force. Some materials are able to pass through the membrane and into a permeate stream, whereas others are not and accumulate in the retentate stream.
Many types of membranes are porous (e.g. micro-porous, nano-porous and ultra filtration membranes). In porous membranes, the pore size and distribution affects the permeability of the membranes to specific particles, molecules or substances. Other types of membranes are not porous (e.g. Reverse Osmosis membranes), and it is the physicochemical properties of such membranes that affect their permeability to specific particles, molecules or substances. Numerous methods have been used to prepare membranes having desired physicochemical properties, pore size and distribution in order to provide desired permeability properties for particular applications.
Membranes are used in membrane separation systems for a wide variety of processes in which it is desired to remove or reduce the concentration of certain materials in a fluid. For example, membrane separation systems have been used to purify waste water effluents from industrial and domestic sources. Membrane separation systems are also increasingly being used to produce fresh water from sea water or brackish water supplies by removing the salt, and other contaminants, from a saline feed.
During operation, the surface of the membrane can become covered with materials present in the feed fluid to which the membrane is highly impermeable, i.e. the membrane rejects the materials, but not the fluid in which they are suspended or dissolved. The flux of fluid permeating through the membrane is then largely devoid of such materials. The materials rejected by the membrane (referred to as the retentate) are typically swept away from the surface of the membrane by maintaining a flow of fluid parallel to the surface (commonly called a cross flow). The retentate (also referred to as the sludge) is collected at the opposite end of the membrane where it is discarded.
Under some circumstances, some of the materials that are present in the sludge at high concentrations become attached, in some circumstances permanently, to the surface of the membrane. This process is known as “fouling”. Fouling leads to a reduced permeate flux and can also lead to a decrease in the rejection characteristics of the membrane. The latter is, for instance, very important in the performance of so called “Reverse Osmosis” membranes for removing salt from sea water or brackish feed water. Fouling in such membranes will give rise to reduced salt rejection and hence lower quality (higher salinity) permeate fluxes.
The replacement or treatment of fouled membranes can be a significant cost in the operation of membrane separation systems. Accordingly, it is desirable to reduce the incidence of fouling in membrane separation systems.